1. Field of the Invention
The present invention is directed to a composite material having a microstructure model with properties to improve fracture toughening, and in particular to a microstructure model including entities that rotate under both an applied load and additional induced loads in order to counteract, deflect and prevent crack propagation from moving throughout the material.
2. Discussion of the Related Art
This invention pertains to subject matter disclosed in U.S. Pat. No. 5,826,213, the text of which is hereby incorporated by reference. The present invention uses principles from materials engineering in which to tailor a microstructure of a material to desirable properties or for a specific device. The invention also utilizes principles of applied mechanics in which to prevent crack propagation through a microstructure based upon mechanical interaction with applied loads. Additionally, this invention uses complex variables to determine the interaction of the microstructure with applied loads based upon principles of applied mathematics as disclosed by N. I. Muskhelishvili, Some Basic Problems of the Mathematical Theory of Elasticity, Fourth Edition, English Translation, P. Noordhoff Ltd., Groningen, The Netherlands, 1963; N. I. Muskhelishvili, Praktisiche Lösung der fundamentalen Randwertaufgaben der Elastizitätstheorie in der Ebene für einige Berandungsformen, Zeitschrift für Angewandte Mathematik und Mechanik, volume 13, pages 264–281 (1933).
Many techniques have been used to enhance material toughening against fracture, including the use of fibers, coatings, laminates, particles and phase transformations in the microstructure of composite materials. Such well-known techniques include composites that contain short cylindrical fibers added to and dispersed within the material, or strips and laminate layers running the entire length of the material used in a device. Other toughened microstructures feature the use of chemical additions and resulting residual stresses of small nanoparticles as disclosed by T. Hirano and K. Niihara, Microstructure and Mechanical Properties of Si3N4/SiC Composites, Materials Letters, volume 22, pages 249–254 (1995); and Koichi Niihara, New Design Concept of Structural Ceramics-Ceramics Nanocomposites, The Centennial Memorial Issue of The Ceramic Society of Japan, volume 99, number 10, pages 974–982 (1991).
The techniques of the prior art rely upon the residual stresses, in microstructures arising either from thermal expansion mismatch during heating and cooling or from phase transformations, to interact with the catastrophic concentrated stresses around a crack tip in order to reduce the chance of crack propagation. Although these residual stresses may be set once fabrication or a phase transformation is completed, they might be attenuated by a large applied load itself before they can negate part of the concentrated stress around a crack tip, thereby making such techniques inefficient. Furthermore, the layup of composites comprising fibers, coating and laminates in lightweight ceramics and intermetallics involves several process steps, thereby increasing the cost of producing the composite.
Another problem with the prior art is that once a crack has passed a particular entity or additive, however, such entity or additive has in most cases lost its ability through unloading to interact with or deflect other cracks. It would be desirable therefore to have a composite material based upon a microstructural model including entities that prevent cracks from moving rather than having the cracks propagate through the material.